1. Field of the Invention
The present invention relates to a plastic molded type semiconductor device and, more particularly, to a semiconductor device including a package having a thin plastic molding layer.
2. Description of the Related Art
In recent years, integrated chip (IC) devices having a high degree of integration have been developed. The IC device is required to be compact. An IC device generally has a shape suitable for surface mounting. In order to cope with the above situation, an IC device having a large number of pins is required in accordance with the remarkable increase in the integration density of elements. Therefore, various developments have been performed.
As a part of the developments, a plastic molding material has been improved. The Bare Chip concept is incorporated in an assembling step of a semiconductor element and especially in a plastic molding step.
A lead frame method, mainly performed as an assembling step of a semiconductor element, has been frequently applied to DIP (Dual In Line Package) type IC devices having a large number of pins or SIP (Single In Line Package) type IC devices or the like.
A lead frame of a given type has a frame obtained by punching or photoetching a thin conductive metal plate and a plurality of leads which extend from the edge to the center of the frame. The distal ends of the leads are free distal ends. An island for mounting a semiconductor chip, such as a semiconductor integrated circuit chip, is formed near the free distal ends of the leads. The island and the plurality of leads are formed by a punching process or a photoetching technique at the same time as the formation of the frame.
The number of semiconductor chips which can be mounted on one island is limited. For this reason, a large number of islands, each having an area in which a semiconductor chip can be mounted are formed, and a lead frame on which a plurality of semiconductor chips can be fixed, is developed. Using this lead frame, a hybrid system module product in which a desired electric circuit is constituted by a plurality of chips is produced.
In an assembling step using a lead frame, a semiconductor chip must be electrically connected to a lead prospectively serving as an inner lead. For this reason, a pad formed on the semiconductor chip and made of a conductive metal plate electrically connected to an active element or a passive element is electrically connected to the lead through a thin metal wire by a bonding method or an ultrasonic bonding method. A flat plate-like lead frame in which a plurality of leads and an island are arranged on the same plane is usually used. In some cases, a lead frame in which a mounting surface of the island for the semiconductor chip is lower than the upper surface of another lead, i.e., a so-called depressed type or island down type lead frame may be used.
FIG. 1 is a conventional IC device using a normal flat plate-like lead frame. Referring to FIG. 1, reference numeral 11 denotes a lead frame. Two islands 12A and 12B are formed on the lead frame 11. Semiconductor chips 14A and 14B are respectively mounted on the islands 12A and 12B through an adhesive 13. Reference numeral 15 denotes a package made of a molded plastic. Note that, in FIG. 1, a thin metal wire connected by a bonding method is omitted.
FIG. 2 shows a conventional IC device using a depressed type or island down type lead frame. In the IC device, reference numeral 11 denotes a lead frame; 12A and 12B, islands; 13, an adhesive; 14A and 14B, semiconductor chips; and 15, a package.
Connection between each pad on a semiconductor chip and the corresponding lead by a thermo-compression bonding method or an ultrasonic thermo-compression bonding method is performed in the following manner. Each metal thin wire held by a bonder serving as a bonding device is compressed and thermally bonded on, e.g., the corresponding pad at the end of a tool at a predetermined temperature in a predetermined atmosphere, and the tool is moved along a predetermined track on the lead to bond the line to the lead by wedge bonding. As a result, a loop-like metal thin wire is connected between both the thermal bonding points.
In the IC device in FIG. 2, a mounting surface of each island is arranged to be uniformly lower than the upper surface of a lead by 0.15 mm to 0.2 mm.
After mounting a semiconductor chip on an island, a package having predetermined outer dimensions is formed through a plastic molding step by a transfer molding method. In this step, a pair of upper and lower metal molds are set on a plastic molding device, a lead frame is stored in a cavity formed in the lower metal mold, and a molten molding plastic is flowed inside the cavity. Arrows in FIGS. 1 and 2 represent paths along which the molten molding plastic flows in the cavity.
There is a semiconductor chip in which a current is supplied in a direction of the thickness of a semiconductor substrate. Electric characteristics of this semiconductor chip is affected by the thickness of the semiconductor substrate. For this reason, in this semiconductor chip, the thickness of the semiconductor substrate is changed in accordance with its characteristics. Therefore, when each chip mounting surface of all islands is uniformly lower than the upper surface of the corresponding lead, the following drawback occurs during a plastic molding step. That is, semiconductor chips having different thicknesses are mounted on a plurality of islands, a lead frame is stored in a cavity, and a molding plastic flows in the cavity. At this time, differences in flow rate and speed of the molten molding plastic between upper and lower portions occur within the cavity with respect to the lead frame as the center. Therefore, unsatisfactory filling of the plastic occurs. This phenomenon becomes conspicuous when the thickness of the plastic molding layer, i.e., the thickness of the package is smaller than 1.5 mm.
This unsatisfactory filling of plastic causes not only degradation of an outer appearance of the package but formation of a void inside the package. The void generates thermal stress and causes the thin metal wires bonded by the thermal bonding method to disconnect. In addition, the void allows entrance of moisture to degrade a moisture resistance.
Thus, the unsatisfactory filling of the plastic causes degradation of reliability of a semiconductor device